The invention described in U.S. Pat. No. 4,289,405 is directed to determining the optical densities of the individual color swatches which make up a color bar, printed on a sheet. The color swatches represent, for instances, the colors of the inks which are being employed in a four color printing operation. The device (and the printed product resulting from its use) described in U.S. Pat. No. 4,289,405 has been very successful but it does have a limitation for certain applications. In using the device described in U.S. Pat. No. 4,289,405, there is a preference that the printing procedure provide a color bar whose height is at least 5 mm. The preference of 5 mm comes into being because the alignment of the color bar, relative to the path of the light reading head, is done "by eye." We have found that there is increased error in this alignment when less time is given over to the alignment procedure. Under heavy production demands, which appear to be the norm, there is a problem in taking sufficient time to properly align the reading head. A misalignment of the color bar relative to the optical scanning path can produce serious errors in the optical density readings. For instance the densitometer could be reading the density of the color bar and/or either "non-printed on" white paper or the color of a piece of artwork that should be printed and happens to be close to the color bar.
The problem becomes more critical with the web presses. In the use of web presses, a small amount of additional paper beyond that require to fold and trim the signatures may represent a very costly annual consumption of paper. As a result, in order to help control the color quality of the printed job, a color bar is used, but generally the color bar can only be 3 mm in height. A color bar of 3 mm height can be fitted into the permissible trim area of the folded signature. Accordingly, no additional paper, other than the normal amount which is trimmed off, is employed. However, the reduced height (i.e. 3 mm) of the color bar causes the alignment problem, described above, to become more critical and it is to such critical alignment that the present method and apparatus is directed.